Wednesday, May 25, 2011

I finished the drafting table

I finished my wife's drafting table, so I'll tell you how I decided to get it to work.

First, I ended up finding a heavy-duty hinge laying around, so I bought another one for the front instead of making my own. It works out alright (probably better than my home-made idea would have), but they aren't perfectly aligned. They're aligned with the front rail, but I used that rail as part of a tripod to hold some rebar I was welding on for my arbor, and bent it when I cut it loose. It doesn't really impede opening the top, but I could tell it was flexing that front rail when I opened the top all the way.

To the right, there's a picture of most of the workings on the handle end of the winch. The winch pipe is a piece of galvanized pipe I had left over from the legs. It is held against the front legs and front rail with a pair of couplers (3 inch, I think) that I bought at Lowe's. I didn't want to drill a hole through the pipe with a hand drill; I don't have any way to hold it straight while I'm doing it or to keep the bit from walking. So I picked up a couple of half-links from Lowe's where they cut the chain. They gave them to me free, since they don't have a use for them. They are the leftovers from when they cut chain to length for a customer. I welded them to the pipe and tied the cable off to that. On the right-hand end, I welded the half-link so that the pipe can't be pulled out far enough for the other end to drop out of the coupler. I didn't measure the distance, but just pulled the pipe out until the end almost dropped, marked the point where the pipe met the right-hand coupler, and then welded the half-link there.

I welded a short piece of rebar across the end of the pipe, and another short piece at a right angle (ok, not-quite-right angle) to that as a handle. I found another piece of pipe (actually, I think it is a "nipple" I bought at Lowe's, and I cut off the threads) to go over the handle. I put a washer on to keep the pipe away from the angle, and then welded another washer on the other end to keep it from coming off. I welded another short piece of rebar sticking out from the coupler as a stop. The handle could be locked behind it to keep the top from coming down.

Then I welded two more hinges on the back rail of the top, and welded pieces of angle-iron to hang down on those and act as lift bars. I drilled a hole in the end of each piece to receive the end of the lift cable. I bought a couple of cheap pulleys from Lowe's. They're pot metal, and I can't weld them, so I ran a nail through the eye and welded it to the back rail of the bottom frame. I then ran a cable from the hole in the bottom of the lift bars, through the pulley, and to the half-link on the winch pipe.

So far, at this point, everything worked, but there were problems. First, the pulleys were mounted behind the lift bars. I didn't take into account the geometry of that setup. The lift bars swung out passed the back before any lift occurred, so they would end up scratching the wall. Also, the pulleys are very loose, so there is a relatively wide gap between the wheel and the carriage. I had to mount the pulleys slightly to one side of the lift bars because they were behind them. I couldn't very well run the cable through the bar. So when the table got close to the top of it's lift, the cable was pulling sideways on the pulleys. The pulleys didn't have enough (or smooth enough) swing to swivel up 90 degrees, so the cable dropped off into the gap between the wheel and the carriage and locked up. So I had to get the pulleys more in line with the lift bars somehow.

Second, my wife kept forgetting to push the pipe back in to lock it, so I needed some way to make it lock itself.

So, for the first issue, I added another cross bar in front of the lift bars. I attached the pulleys to that. But I still hadn't worked out the geometry, so the new crossbar is too far forward, and the lift bars swung way too far forward. I moved the crossbar further back, but I didn't move it all the way against the lift bars, and they still swung too far out. I don't remember why I didn't move them all the way back. I think I was trying to figure out where the lift bars end up to get the top as high as possible, but I'm not sure.

With the lift bars swinging, it cost too much in lift force. My wife couldn't turn it at all with 1 hand, and couldn't do it easily enough even with two hands. So I decided to ditch the pulleys altogether. I got a couple of more half-links and welded them in place of the pulleys. Then I drilled a couple of shorter sections of angle iron and welded them between the back cross bar and the back rail. I ran an eye bolt through each hole to serve as an adjustment. I then ran longer cable from the winch pipe, through the half-link, down to the lift bar (oh, I almost forgot, I welded another half-link at the bottom of each lift bar, as the friction of going through the hole I drilled would have been too much), back up to the half-link, and then to the eye bolt. Double the lift force, but with the added friction of the half-link instead of the pulley. It did get a little easier, but not enough. I finally ended up welding the hinges for the support bars so that they didn't swing at all. That finally made it easy enough that my wife could work with it.

So then I started working on getting it to lock on it's own. I had a piece of angle iron laying around, one of those pieces that have holes and slots all the way down it. I cut a section out of it with the angle grinder and welded this over the end of the left-hand pipe support, so that one of the pre-drilled holes was centered. It wasn't big enough for half-inch rebar to pass through, so I wallowed it out with the hand drill. I then found a large washer I had laying around that just happened to fit over the end of the pipe and had a half-inch hole. I welded it to the end of the winch pipe (I had to cut off the right-hand half-link to get it out first), then welded a piece of rebar to that. I stuck the rebar through the centered hole. Then I bought a compression spring from Lowe's, and put it over the rebar and then put a washer on to hold the spring. I put a clamp on the outside end of the rebar so I could fine-tune the tension before I welded it in place, and brought my wife out to test it. If the spring was tight enough to ensure that the crank handle was pulled back behind the stop every time, it was too tight for my wife to pull the handle out and still crank it with one hand. And even when she used two hands, the desk was so light that she was lifting it off the ground instead of actually turning the handle. I noticed that it was harder to pull than it should have been because the ridges in the rebar were catching on the angle iron, so I ground those out smooth. But that wasn't enough to affect it much; it just made it a smoother pull. So the last thing I added was an extra piece of metal that I cut off of the angle with the holes. I welded it so that it formed a ramp from the edge of the winch pipe support coupler to the outer end of the rebar stop. This way, when my wife cranks the handle, it slides up the slope and passed the stop without her having to pull it out by hand. She can now crank it with one hand, and hold down the desk if she needs to.

Once I got that in place, I welded the washer holding the spring into place where the clamp had been holding it, and cut off the extra rebar. I cut a piece of 3/4" MDF for the top and attached a scrap of wood down the front edge. My wife painted the table with some chrome paint we had laying around, and painted the top with white. It makes an awful lot of noise, and looks like it takes her a lot of effort to crank, although it's easy enough for me. She thought all the mechanics were a waste of time, until we got it into her art room and she tilted it up for the first time to work on some project. Then she loved it. It's bigger than anything she could buy from hobby lobby, and still cheaper overall.

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